GSM is a set of ETSI (European Telecommunications Standards Institute) standards specifying the infrastructure for a digital cellular service. Subsequent to its commercial operation in the European countries in 1991, GSM rapidly gained acceptance and market share worldwide. In addition to digital transmission, GSM incorporates many advanced services and features, including ISDN (Integrated Services Digital Network) compatibility and worldwide roaming in other GSM networks. The functional architecture of a GSM system, in general, includes the mobile station (MS), the Base Station subsystem, and the network subsystem. Each of these subsystems comprises functional entities that communicate through various interfaces using specified protocols.
The mobile station in GSM comprises the actual hardware, which is the mobile equipment, and the subscriber information, which includes a unique identifier called the International Mobile Subscriber Identity (IMSI). IMSI is stored in the Subscriber Identity Module (SIM), implemented as a smart card. The SIM card may be placed in any GSM mobile equipment to make and receive calls at that terminal and to receive other subscribed services.
The Base Station Subsystem is composed of the Base Transceiver Station (BTS) and the Base Station Controller (BSC). The BTS houses the radio transceivers that define a cell and handle the radio interface protocols with the mobile station. The BSC manages the radio resources for one or more BTSs and the radio interface channels (setup, tear down, frequency hopping, etc.), as well as the handovers.
The Network Subsystem includes four intelligent databases and the Mobile services Switching Center (MSC), which is central to the Network Subsystem. While acting as a normal switching node of the PSTN (Public Switched Telephone Network) or ISDN, it provides all the functionality needed for handling a mobile subscriber, including registration, authentication, location updating, inter-MSC handovers, and call routing to a roaming subscriber. The MSC also provides the connection to the public fixed networks.
One of the four intelligent databases included in the Network Subsystem is the Home Location Register (HLR), which contains all the administrative information of each subscriber registered in the corresponding GSM network, along with the current location of the subscriber. There is logically one HLR per GSM network, although it may be implemented as a distributed database. The second intelligent database is the Visitor Location Register (VLR). The VLR contains selected administrative information7 from the HLR, for each mobile subscriber currently located in the geographical area controlled by the VLR, which is necessary for call control and stipulation of the subscribed services.
The other two intelligent databases are used for authentication and security purposes. One of them is the Equipment Identity Register (EIR), which contains a list of all valid mobile equipment on the network, where each mobile equipment is identified by its International Mobile Equipment Identity (IMEI). An IMEI is marked invalid if it has been reported stolen or is not type approved. The other intelligent database is the Authentication Center (AuC), which is a protected database that stores a copy of the secret key stored in each subscriber's SIM card.
The available GSM radio spectrum is divided into 200 kHz carrier frequencies using FDMA (Frequency Division Multiple Access). One or more carrier frequencies are assigned to individual base stations, where each carrier is divided into eight time slots using TDMA (Time Division Multiple Access) and where eight consecutive time slots form TDMA frames. A transmission channel occupies one time slot within a TDMA frame. TDMA frames of a particular carrier frequency are numbered, and both the mobile station and the base station are synchronized on this number. Larger frames are formed from larger groups of TDMA frames. The position within such frames defines the type and function of a channel.
Common channels can be accessed both by idle mode mobiles in order to change to dedicated mode, and by dedicated mode mobiles to monitor surrounding base stations for handover information. The common channels include:                BCCH (Broadcast Control Channel), which continually broadcasts, on the downlink, the information such as the base station identity, frequency allocations, and frequency-hopping sequences.        FCCH (Frequency Correction Channel) and SCH (Synchronization Channel), which synchronize the mobile to the time slot structure of a cell by defining the beginning of a TDMA frame.        RACH (Random Access Channel), which is used by the mobile to request access to the network.        PCH (Paging Channel), which alerts the mobile station of an incoming call.        AGCH (Access Grant Channel), which allocates an SDCCH (Stand-alone Dedicated Control Channel) to a mobile for signaling (in order to obtain a dedicated channel), following a request on the RACH.        
Each PCH block message is composed of four transmission “bursts.” A PCH message is scattered among the four bursts as a result of the fire coding, the convolutional coding, the interleaving, and the mapping process, all prior to transmission of the message. FIG. 1 is a block diagram depicting these processes. If a PCH message is a dummy one, there is no need for a handset to turn on and analyze its four carrier bursts; however, the handset needs to make a determination as to whether the PCH message is a dummy one before deciding not to receive the rest of it. Therefore, a handset saves power if it can quickly make such a determination and ignore the remainder of a dummy PCH block.